Return `Result` from all `generate_*` methods #2369

Job	Run time
build (1.65.0, thumbv7em-none-eabi)	3m 52s
build (1.65.0, wasm32-unknown-unknown)	6m 1s
doc	31s
build (stable, thumbv7em-none-eabi)	33s
build (stable, wasm32-unknown-unknown)	52s
minimal-versions	41s
test (1.65.0)	3m 42s
test (stable)	25s
test (nightly)	28s
	17m 5s

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